Door knob adapter



FIG. 1 is a front elevation view of a door knob adapter showing my new design;

FIG. 2 is a rear elevation view thereof;

FIG. 3 is a right side elevation view thereof;

FIG. 4 is a left side elevation view thereof;

FIG. 5 is a top plan view thereof; and,

FIG. 6 is a bottom plan view thereof. 

The ornamental design for a door knob adapter, as shown and described. 